We report here the synthesis of yttrium-aluminum garnet oxide (Y 3 Al 5 O 12 ) nanopowders by liquid-feed flame spray pyrolysis (LF-FSP) of combinations of yttrium and aluminum precursors dissolved in EtOH, n BuOH, and/or THF. These include solutions of the following: yttrium and aluminum nitrates in EtOH or n BuOH; yttrium 2-ethylhexanoate and alumatrane [N(CH 2 CH 2 O) 3 Al] in THF or EtOH; yttrium methoxyacetate and alumatrane in EtOH; yttrium acetylacetonate and alumatrane in EtOH, and yttrium propionate and aluminum acetylacetonate in EtOH or THF. Each precursor system was aerosolized with O 2 and subsequently ignited. Following combustion, the resulting powders were collected by electrostatic precipitation at rates of 50 g/h. Surprisingly, the precursor choice strongly influences both the initial phase composition and morphology of the LF-FSP powder, as well as the phase changes that occur during annealing. As-collected LF-FSP nanopowders, average particle size (APS) e100 nm, had the YAG composition of the precursor feed; but XRD shows an apparent mixture of hexagonal YAlO 3 I and some Y 4 Al 2 O 9 (YAM). The remaining Al 2 O 3 exists either as nanosegregated, amorphous alumina or in defect structures. However, the most homogeneous powders exhibit FTIR, TGA/DTA, TEM, and XRD data that suggest a new phase with a modified YAlO 3 I crystal structure and a YAG composition. Powders annealed at 900-1000 °C (7-10 d) transform without grain growth or necking to free-flowing YAG phase powders. The activation energy for this phase transformation was ≈100 kJ/mol, much lower than values reported for amorphous Y 3 Al 5 O 12 .

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The flame spray pyrolysis of alcohol-soluble precursors allows the synthesis of mullite-composition nanopowders (average size of ϳ60 -100 nm) that, when annealed carefully, provide processable nano-mullite powders. The powders have been characterized using several spectroscopic and microscopy methods, including thermal gravimetric analysis, differential thermal analysis, diffuse reflectance infrared Fourier transform spectroscopy, and transmission electron microscopy. Preliminary studies on the pressureless-sintering behavior of these powders are presented. Without additives or any efforts to optimize the process, powder compacts could be sintered to relative densities of >90%, with grain sizes of <500 nm at 1600°C.